whyaretwo. Welcome to GTTalk. There are a lot of things to consider when looking to buy a tractor for "daily" use. The condition is, of course, a major factor. The suitability of the model for a the desired task (which is usually determined by the experiences of others, but is largely an opinion), and last but not by any means least is the availability of parts.
I'm not familiar enough with the models that you mentioned above to formulate an educated opinion, but here's what I know.
I'm not familiar with the Simplicity at all, but the age could make it a potential parts problem.
The AC B-1 was the first garden tractor offered under the AC name. I believe that it was quickly replaced by the B-10, making it a short run tractor and therefore a potential parts problem also.
The AC 712S is a much newer machine than the other two. I believe that it is also much larger. Parts availability may be OK for this model as I think they were marketed under other manufacturers names as well (such as Simplicity) and some parts would be interchangeable. AC and Simplicity are still linked to this day as both of those brands are now manufactured by Snapper, although I've never checked the availability of parts for any of these models from the dealer.
I know that this wasn't of any specific help, but I think that it's good food for thought.
Some of our experts will be along in short order to help out I'm sure. Thanks for signing up!

My question is based on the assumption that all of the tractors are in relatively equivalent condition. I suspect any of the tractors will be overkill for the lion's share of the tasks asked of them, primarily mowing a medium sized suburban lawn and plowing a relatively short driveway. I can accept that with a vintage machine I probably won't be walking in to the local mower dealer or tractor supply for certain machine specific parts, but what I want to avoid is a tractor with a known, failure-prone, proprietary component. I don't want to be bidding against the entire GT community for one of the last parts that everyone will need to keep mower X working.

For me, the more common standard components the better. I don't have any experience with GT's but I worked as a mechanic for years so I'm comfortable doing pretty much any repairs myself.

My yard is actually small enough to push mow, if you've got the time. Takes about 1.5 hours. I no longer want to spend the time it takes though. As a compromise to moving to a house with a smaller yard I'm getting a rider. I want a tractor versus a dedicated mower because they're statistically more reliable and the ability to add attachments (such as a snow blade (cardiologist has said no more shoveling snow, ever)) increases the utility of the purchase. I want a vintage tractor because in my opinion they're cool. It feels a bit silly to use a rider on a lawn this small so if I'm going to use a rider on a lawn that doesn't honestly need one I can at least use one that makes me happy because it's cool.

Since I'm using the GT for a task that will in no way tax it, with care, proper maintainence, and maybe a bit of luck, it should last me the rest of my (hopefully long) life. Unless I manage to buy one with an oddball part that breaks and either can't be replace or costs a fortune to replace because you have to have a new one custom machined.

I know that the evenings refreshments have kicked in and I'm rambling a bit , but I just wanted to give a bit more info on where I'm coming from as a potential GT buyer.

As an aside, not a Simplicity or A-C, I found what looks to be a very nice Sears super 12 with deck, blade and dualies that I think might make a gorgeous bit of overkill puttering around my little suburban lawn.

One thing I would look at is what motor it has. Tecumseh parts can be very expensive to replace. I have seen used SSI ignition coils run over $200 on eBay. I was bidding on a NOS carb for a HH120 that sold for $225. I found the parts I need to rebuild my HH120 but I don't want to float a loan to finish it, to replace both valves would cost around $100. The exhaust valve alone is about $75!!! A new piston with rings and pin is close to $100. I have considered buying used parts for the motor. The Sears should have a Tecumseh motor, the others I believe have Kohlers. Kohler parts are easier to find, their are enough pullers out there to keep the prices down.

As far as the rest of the parts, most of these use parts from the same companies, so the interchange of parts aren't so bad.

I would go with the 712S myself. More used and new parts available. We have a Big Ten and a B112. The Big Ten is equivalent to a landlord and the B112 is a sovereign. While any of the 3 you listed will be good tractors the round hood models seem to be more sought after by collectors than the newer ones. I have a 48" deck, 42" blower and 42" tiller for the B112 and it has a happy semi-retired life here with my CASE 195 and Ingersoll 446.

Thanks for the replies! Sadly both of the Allis-Chalmers were gone by the time the opportunity to actually go look at them occurred. Which left me with the choice between the 61 Simplicity and the 68 Sears. Ultimately I chose the Sears. It came with a deck, blade, duals, 3 point harness and tire chains. The Simplicity was just the tractor. You never know though, there could still be a Simplicity or AC in my future.

Thanks for the update. How about some pics of your new tractor? The Sears should still be a good machine for you to use. Some of the older parts are getting harder to find is all. Best of luck with it and keep us posted!

At the moment it's sitting on a trailer at a buddy's house. As soon as I get home and can work out a time for him to bring it over I'll be posting some large, detailed pics. For now I've put up the pics the seller texted to me in a post over in the Sears/Roper forum.

These Sears have a bulletproof rear end, and I think this one has hi-lo like the SS & GT line (I've never owned an ST model). The only weak link in the whole machine is getting parts for the engine. If that ever becomes a problem, it is very easy to install a newer engine into this tractor.